Silicon sputtering target with special surface treatment and good particle performance and methods of making the same

ABSTRACT

A sputter target assembly comprising a Si target and a backing plate is provided wherein the backing plate is bonded to the target. The Si target comprises a smooth, mirror-like surface and has a surface roughness of less than about 15.0 Angstroms. Methods are provided for producing silicon target/backing plate assemblies wherein a silicon blank is processed to remove scratches from the blank surface resulting in a mirror like surface on the target, and a surface roughness of 15.0 Angstroms or less. The method comprises a first and second cleaning step with the first step being performed before the scratch removal step, and the second step being performed after the scratch removal.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the priority benefit of U.S. Provisional Patent Application Ser. No. 61/556,926 filed Nov. 8, 2011.

FIELD OF INVENTION

The invention relates to a silicon sputtering target bonded to a backing plate consisting of Mo, Ti, Zr, Ta, Hf, Nb, W, Cu and the alloys of these elements or Mo/Cu, Ti/Al composite backing plates, and to a method of producing such a target. One aspect of the invention relates to a silicon-backing plate sputtering target assembly with a special surface treatment.

BACKGROUND OF THE INVENTION

Silicon (Si) is one of most common elements in the universe by mass and it most exists in compound forms of sands, dusts, silicon dioxide (silica), or silicates, etc. Pure crystal silicon is a gray color and brittle in nature. Silicon has atomic number 14, an atomic weight of 28.09, and a density of 2.33 g/cm³. Silicon has a diamond cubic crystal structure at room temperature, a melting point of 1310° C., a boiling point of 2357° C. Furthermore, silicon has a linear coefficient of thermal expansion (CTE) of 2.49 μm/m-° C. at temperature 25.0° C.

Silicon is a semiconductor and plays a key role in the modern world economy. In fact, the entire modern semiconductor microelectronics industry is established on silicon base. Silicon is widely used for integrated circuits, chips, logic electronics devices, and memory electronics devices. Silicon and its compounds are not only used for forming the substrate where the semiconductor chips are built on but also for the function units or layer for transistors and stacking structures such as silicon electrode, silicon dioxide dielectric layers, and silicon nitride mask layers, inside the chips and integrated circuits (IC).

In recent years, high-k metal gate (HKMG) transistor technology has been developed and applied to 45 nm and below to manufacture IC devices for a wide variety of high performance and low power applications such as graphics, networking, and wireless mobile applications. One of the key tasks for high-k metal gate (HKMG) is to find out suitable new metal materials and reliable film formation methods to control of the channel with high on currents and low leakage current and keep the integrity of complex structures. Hf, TiAl, TiN, TiAlN, TiSiN, TaAlN, TaSiN, and rare earth metals are reported to be used as metal gate materials. Recently, much attention has been paid to utilize silicon to as metal gate structure materials due to its unique chemical and physical properties and natural connection to the semiconductor technologies built on the silicon.

Silicon and silicon nitride layers beyond the substrate inside the IC and chips are traditionally formed through chemical vapor deposition (CVD) method. As the microelectronics industry drives the miniaturization of devices and circuits towards nanometer dimension utilizing 45 nm and below technologies, ever-increasing stringent demands have been placed on the precision and minimum impact device integrity of film/structure formation methods. Sputtering is a mechanism by which atoms are removed from the surface of a material (target) as a result of collision with high-energy ions through a physical vapor deposition (PVD) technique wherein atoms or molecules are ejected from a target material by high-energy ions bombardment so that the ejected atoms or atom clusters can condense on a substrate as a thin film. Sputtering has more precise control of the transportation and deposition of mass atoms and has less thermal impact on the deposited film structure than a CVD process. Si sputtering target through a PVD process is becoming more widely used to silicon layers such metal gate electrode and its compound structure layers such as silicon nitride and silicon carbides inside microelectronics integrated circuits and communication devices for a wide variety of high performance and low power applications such as graphics, networking, and wireless mobile applications.

SUMMARY OF THE INVENTION

In one embodiment, a silicon target comprises a Si target blank with a defect-free surface and molybdenum backing plate, molybdenum copper composite backing plate, or titanium aluminum composite backing plate. The target blank is bonded to backing plate through solder bonded, braze bonded, foil bonded, or other low temperature bonding methods. A surface process has been developed to achieve a nearly defect/damage-free surface for silicon sputtering target to reduce the burn-in time and particles on the films deposited from the sputtering targets. The surface process comprises the steps of machining, grinding, inspection, lapping, cleaning, surface damage removal (i.e., scratch removal), cleaning, polishing, cleaning, inspection, final cleaning, and final inspection. This invention also provides methods of manufacturing silicon sputtering targets.

BRIEF DESCRIPTION OF THE DRAWINGS

The below detailed description makes reference to the accompanying figures, in which:

FIG. 1 is process flow chart of a comparative Si target surface process A.

FIG. 2 is process flow chart of a Si target surface process in accordance with one embodiment of the invention, namely process B.

FIG. 3 is a group of optical micrographs of the surfaces of Si targets made by comparative process A. Defects including scratches, micro-cracks, indentations, pores, and chips were observed on these surfaces.

FIG. 4 is a group of SEM images of the surfaces of Si targets made by comparative process A. Defects including scratches, stains, pores, chips, and micro-cracks were observed on these surfaces.

FIG. 5 is a group of optical micrographs of the surfaces of Si targets made by process B in accordance with one embodiment of the invention. These surfaces are clean and almost defect-free except for few spot defects.

FIG. 6 is interferometer imaging and surface roughness measurement results of the Si target made by comparative surface process A. Parallel scratches were detected at target center. Interferometer roughness has been performed at five positions—target surface center (PC), 3 o'clock position (P3), 6 o'clock position (P6), 9 o'clock position (P9), and 12 o'clock position (P12) 3 inches away from target surface perimeter. The measured roughness is PC=20.848 Angstrom, P3=16.749 Angstrom, P6=14.689 Angstrom, P9=16.199 Angstrom, and P12=15.487 Angstrom. The average measured roughness is 16.794 Angstrom and a standard deviation of 2.394 Angstrom.

FIG. 7 is interferometer imaging and surface roughness measurement results of the Si target made by surface process B in accordance with one embodiment of the invention. No apparent defects were detected on the surfaces. The measured roughness is PC=11.911 Angstrom, P3=13.857 Angstrom, P6=15.079 Angstrom, P9=14.625 Angstrom, and P12=11.888 Angstrom. The average measured roughness is 13.472 Angstrom and a standard deviation of 1.500 Angstrom.

FIG. 8 is particle map and particle counts of Si films deposited from the Si target made by comparative process A. A total of 869 particles have been detected on the films after the Si target had been conditioned/burned in for more than 24 hours.

FIG. 9 is particle map and particle counts of Si films deposited from the Si target made by process B in accordance with one embodiment of the invention. Only 25 particles have been detected on the films after the Si target had been conditioned/burned in for less than 8 hours.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The silicon sputtering target encompassed by this invention can have any suitable geometry. The present invention includes a method of manufacturing the silicon target with a scratch removal surface treatment and good particle and burn-in performance. The silicon target has a purity of at least 99.999% and preferably 99.9999% excluding dopants. The silicon target blank has diameter up to 550 mm and can be intrinsic, p-type doped, or n-type doped. The silicon blank can have polycrystalline, single crystal, or semi-single crystal structure. Although other structures of Si can be used, the single crystal structure is preferable according to this patent. The silicon raw material and its dopant elements are melted to grow a silicon ingot from a molten liquid preferably through Czochralski crystal growth method or casting process. The resulting ingot can have any size and any suitable shapes including round, square, and rectangular. The silicon ingot is subsequently inspected and wire sawed into ingot slices of different desirable thicknesses for making different parts. The silicon ingot slices undergo a surface process treatment to achieve desirable dimension and surface condition. The surface process includes but is not limited to manufacturing operations such as machining, grinding, lapping, polishing, surface damage removal (i.e., scratch removal), etching, cleaning, and inspection, preferably additional cleaning steps would be applied between these any two these operations, to form a Si target blank with desirable surface conditions and characteristics and geometry dimensions. The finished Si blank is then bonded to a backing plate through solder bond or other low temperature bond methods to obtain a silicon target blank-backing plate assembly. The solder can be but not limited to indium, tin-silver, and also can use nanofoil. There are many backing plate materials options include but are not limited to Mo, Ti, Zr, Ta, Hf, Nb W, Cu and the alloys of these elements or composite backing plates like Mo/Cu and Ti/Al. The backing plate materials have a coefficient of thermal expansion (CTE) that is chosen to closely match silicon's CTE. The bonded target assemblies can have diameters of 200 mm, 300 mm, and more and can be made for target configurations with different OEM designs.

An exemplary silicon target with superfine surface has been produced according to the processes described above. Si raw material of 99.9999% or higher purity is used to grow an Si ingot, preferable dislocation-free single crystal Si ingot with a diameter from 300 mm to 800 mm, preferable diameter of greater than 450 mm by use of the Czochralski crystal growth method. The composition of the resulting ingot measured by the GDMS method is listed in the Table 1.

TABLE 1 Impurity element Content (ppm) Impurity element Content (ppm) S 0.092 Ge 0.05 Li 0.005 As 0.005 Be 0.005 Rb 0.05 B 0.019 Zr 0.02 F 0.05 Mo 0.02 Na 0.01 Pd 0.02 Mg 0.01 Ag 0.01 Al 0.07 Cd 0.03 P 0.01 In 0.02 Cl 0.48 Sn 0.05 K 0.025 Sb 0.01 Ca 0.05 Cs 0.005 Ti 0.002 Ce 0.02 V 0.002 W 0.02 Cr 0.01 Pt 0.01 Mn 0.005 Au 0.02 Fe 0.065 Hg 0.01 Co 0.005 Tl 0.005 Ni 0.02 Pb 0.02 Cu 0.051 Th 0.001 Zn 0.05 U 0.001 Ga 0.01 C 1.54 O 2.03 N 26.5

The weight concentration unit is ppm for all elements and total metallic impurities contents are less than 1 ppm. The ingot is sawed into ingot slices of desirable thickness. The ingot slices are subsequently subjected to process A to achieve finish surfaces and dimensions. Comparative process A as herein referred to consists of manufacturing operation steps machining, grinding, lapping, polishing, cleaning, and final inspection. Process B in accordance with one aspect of the invention consists of manufacturing operation steps machining, grinding, inspection, lapping, cleaning, surface damage removal, cleaning, polishing, cleaning, inspection, final cleaning, and final inspection. The cleaning agents include but are not limited to de-ionized water and or acids. Further cleaning agents and cleaning apparatii may be chosen from U.S. Pat. No. 8,227,394 (Zhu et al.) herein incorporated by reference. As is stated in the '394 patent, a cleaning fluid, such as a gas or liquid, can be prepared that may include a buffering agent and one or more polymers. The polymers are solubilized in the cleaning fluid and have a molecular weight of greater than about 10,000 g/mol. The polymers may, for example, be chosen from polyacrylic acid (PAA), polyacrylamide (PAM), hydroxyethyl cellulose (HEC). Copolymers of PAA and PAM are also mentioned as being exemplary. The weight percent of the polymers in the cleaning fluid may be about 0.001-10% by weight.

In addition to water, other polar solvents such as isopropyl alcohol (IPA), dimethyl sulfoxide (DMSO), and dimethyl formamide (DMF) may be mentioned. Weak acids and weak bases such as citric acid and ammonium (NH₄OH) can be mentioned as exemplary buffering agents.

In accordance with one aspect of process B, the scratch removal step comprises polishing to result in a substrate having an average measured roughness of 15.0 Angstroms or less.

The finished Si target blanks are bonded to backing plate through indium solder bond or low temperature bond methods to obtain a silicon target blank-backing plate assembly. There are many backing plate materials options include but are not limited to Mo, Ti, Zr, Ta, Hf, Nb, W, Cu and the alloys of these elements or Tosoh patented composite backing plates like Mo/Cu and Ti/Al. The materials have coefficient of thermal expansion (CTE) close to match silicon's CTE. The bonded target assemblies can have different 200 mm, 300 mm, and beyond target configurations with different OEM designs. In this case the backing plate material is Mo/Cu composite and the dimension is 300 mm type target.

Surface microstructures examinations have been performed on the silicon targets subjected to surface process A and B. As shown in FIG. 3, the optical microscopy imaging indicates Si targets subjected to comparative surface process A have a significant amount of surface defects including scratches, micro-cracks, indentations, pores, and chips. As shown in FIG. 4, scanning electron microscopy (SEM) confirms scratches, stains, pores, chips, and micro-cracks surfaces exist on the surfaces of the silicon targets subjected to process A. By the contrast, as shown in FIG. 5, the surfaces of the silicon targets subjected to process B are clean and nearly defect-free.

Non-destructive interferometer surface roughness measurements have been conducted to measure surface roughness of the silicon targets subjected to surface processes A and B. FIG. 6 is the interferometer images and roughness measurement data for the surface of a Si target subjected to process A. Roughness has been performed at 5 positions—target surface center (PC), 3 o'clock position (P3), 6 o'clock position (P6), 9 o'clock position (P9), and 12 o'clock position (P12) 3 inches away from target surface perimeter. The measurement indicated average roughness is 16.79 Angstrom with a standard deviation of 2.394 Angstrom. Parallel type scratches were detected. FIG. 7 is the interferometer images and roughness measurement data for the surface of a Si target subjected to process B. No defects were detected on the target surfaces. The measured roughness is PC=11.911 Angstrom, P3=13.857 Angstrom, P6=15.079 Angstrom, P9=14.625 Angstrom, and P12=11.888 Angstrom. The average measured roughness is 13.472 Angstrom and a standard deviation of 1.500 Angstrom. The Si target surfaces subjected to process B have fewer surface defects and lower roughness than the Si target surfaces subjected to process A.

Sputtering tests have been performed on these two 300 mm Si targets made by the process A and B. The sputtering process parameters were identical. The target were conditioned or burned in to get achieve desirable low particle counts. For the Si target subjected to process A, the initial particle count was more than 2000 and as shown in FIG. 8, after 24 hours burning in, there were still over 800 particles detected on the films. In contrast, initial particle count was low for the films deposited from the target subjected to process B, and as shown in FIG. 9, the particle count level was brought down to 50 particles or lower under less than 8 hour burn-in. In some cases, the particle counts detected on the films deposited from the Si target subjected to process B were less than 5, this is one of the lowest particle counts has been ever achieved for the films deposited from the sputtering targets of different materials. The Si targets subjected to process B have significantly lower particle counts and shorter burn-in times than the Si targets subjected to process A.

We discovered that the Si target burn-in time and particle performance were correlated to the target surface conditions or processes. We discovered the Si targets subjected to a surface process (Process B) consisting of surface removal, i.e., scratch removal, and multiple cleaning manufacturing operations have fewer surface defects and lower surface roughness than the Si targets subjected to a surface process (Process A) without target surface damage removal and intermediate cleaning manufacturing operations. We believe process B remove Si target surface sub-layer damages and potential contaminations and thus resulting cleaner target surfaces and fewer surface defects, and fewer film particle counts and shorter burn-in times.

It is apparent then, that methods are disclosed herein for manufacturing silicon sputtering targets. In one embodiment, a silicon sputtering target is provided in conjunction with a backing plate comprising molybdenum or a molybdenum copper composite. The silicon target diameters can range from about 800 mm or less.

The silicon blank has a high surface finish to achieve good sputter performance and is made from a silicon ingot subjected to manufacturing operations, such as sawing, machining, grinding, lapping, polishing, surface damage removal (i.e., scratch removal), etching, cleaning, and inspection. In preferred embodiments, additional cleaning and etching steps are provided between any two of these operations to form a silicon target surface having desirable surface conditions. The resulting silicon targets have a visually mirror shining reflection and nearly damage/defect free surface, with a surface roughness of less than 500 Angstroms, preferably less than 100 Angstroms.

Silicon targets in accordance with the invention have a short burning time and the films deposited from the resulting Si targets have low particle counts.

The silicon target may, for instance, have a purity of at least about 99.999% excluding dopants. The silicon target blank may have a diameter up to about 500 mm and can be intrinsic p-type doped, or n-type doped. The silicon blank can have polycrystalline, single crystal, or semi-single crystal structure.

The backing plate can be pure molybdenum with a purity of 2N5 or higher, or in other embodiments, a molybdenum copper composite backing plate may be utilized. In still other aspects of the invention, a titanium aluminum composite may be used as the backing structure.

The target blank may be bonded to the backing plate through solder bonding such as by use of indium, tin-silver, and nanofoil. In other embodiments, the backing plate materials have a coefficient of thermal expansion that is chosen to closely match the coefficient of thermal expansion of the silicon target material. Such backing plate materials include, but are not limited to, No, Ti, Zr, Ta, Hf, Nb, W, Cu and alloys of these elements, especially Mo/Cu and Ti/Al composite backing plates.

The present invention has been disclosed in connection with the preferred embodiments thereof, it should be understood that the invention is not limited to the specific embodiments described since the means herein comprise preferred forms of putting the invention into effect, and other embodiments may be within the scope of the invention as defined by the following claims. 

What is claimed is:
 1. Sputter target assembly comprising a Si target and a backing plate bonded to said target, said Si target comprising a smooth mirror surface and a surface roughness of less than 15.0 Angstroms.
 2. Assembly as recited in claim 1 wherein said backing plate comprises Mo or a Mo/Cu composite or Al/Ti composite.
 3. Assembly as recited in claim 1 wherein said roughness is about 13.5 Angstroms with a standard deviation of about 1.5 Angstroms.
 4. Assembly as recited in claim 1 in combination with a Si film made by sputter coating of said target upon a substrate, said substrate comprising Si with a particle count of 50 or less after a target burn in of 8 hours or less.
 5. Method of making a silicon target/backing plate assembly comprising providing a Si blank and the step of removing scratches from said blank so as to provide a mirror like surface on said target and a surface roughness of 15.0 Angstroms or less on said surface, said method further comprising a first and second cleaning step, said first step performed before said scratch removal and said second steps being performed after said scratch removal.
 6. Method as recited in claim 5 wherein either said first or second cleaning steps comprises contacting said blank surface with a liquid cleaning solution comprising a polymer.
 7. Method as recited in claim 6 wherein said polymer is a member selected from polyacrylic acid, polyacrylamide, and hydroxyethylcellulose. 